It finds an application notably in turbofan engines and turboprop engines (including a non-ducted fan, generally designated by the English expressions “open rotors” or “propfans”) which can be used for the propulsion of aircraft. Turboprop engines are distinguished from turbofan engines by the presence of unducted propeller blades. Turbofan engines, or “turbofans” according to the English expression, include a ducted fan acting as a propeller. The disclosure herein is applicable notably to these types of turbine engines.
Turbine engines for the propulsion of aircraft may also be designated simply by the term “engine”. In the present document, the term engine thus corresponds to a turbine engine for the propulsion of an aircraft.
These turbine engines utilize one or a plurality of gas turbines recovering the energy produced in a combustion chamber supplied with air by one or a plurality of compressors.
A known phenomenon that can occur at the level of a compressor during the operation of turbine engines is the phenomenon referred to as “surge”. This phenomenon is explained below by the example of a turbofan engine of an aircraft propulsion system, although it is identical in a turboprop engine (or in general in any compressor of a turbine engine).
The phenomenon of surge corresponds to aerodynamic stalling of the vanes of a compressor.
It corresponds to a tendency of the high-pressure zone of a compressor to flow back towards the low-pressure zone. This leads to instability of an oscillatory nature in the rate of flow of the compressor.
On the whole, care should be taken to prevent this phenomenon from occurring, since the variations in or the inversions of the rate of flow can cause reductions in the performance of the turbofan engine or turboprop engine.
It is thus customary to control the operation of the one (or more) compressors of a turbofan engine in order not to cause it to operate within a range of points of operation in which a surge might occur, or in proximity to this range, which is also referred to as the “surge zone”. In other words, for a given rate of flow, it is necessary to satisfy oneself that the pressure ratio of the compressor (or its rate of compression) does not exceed a certain limit or, conversely, that a sufficient rate of flow is present in the compressor according to its pressure ratio or its load. In practice, there is a tendency to cause the compressor to operate at a maximum admissible pressure ratio, in order to ensure good efficiency of the turbine engine.
In the case of a turbofan engine or turboprop engine of an aircraft, the charge of the turbo compressor depends on the power supplied by the turbine engine for the propulsion of the aircraft, but also on takeoffs of mechanical power or air, for functions other than the propulsion of the aircraft.
The takeoffs may be of two kinds: mechanical, and/or of air.
Takeoff (of mechanical energy) corresponds to takeoff of power, via a mechanical transmission, at the level of a drive shaft between a turbine and a compressor. For example, this permits the generation of hydraulic power for the hydraulic equipment of the aircraft, and/or the driving of one or a plurality of generators for the supply of electrical energy.
The takeoff of air (generally designated in the aeronautical field by the English expression “bleed air”) involves taking off a part of the air compressed by the compressor in order to supply, for example, the deicing systems, the air conditioning system, the cargo cooling system, the fuel inerting system, and/or the system for internal cooling of the engines.
These takeoffs impact on the margin in relation to the surge zone. A takeoff of mechanical power, generally taken on the high-pressure shaft, brings the point of operation of the compressor closer the surge zone of the turbofan engine or turboprop engine. A takeoff of air basically tends to increase the flow (mass flow of air) in the compressor, whereas the effect of the takeoffs of air on the pressure ratio is in general negligible, which moves the point of operation of the compressor away from the surge zone.
The most critical point of operation of an engine with regard to surge is at its rate (or “speed”) of idling. In fact, at the lowest speed of the engine, the compressor is driven at a low speed and clearly has a low pressure ratio, but the flow of air in the compressor is also extremely reduced, so that a surge may occur at a low pressure ratio. The margin offered in terms of the pressure ratio when idling is thus very small.
A plurality of approaches are known in order to increase the distance and to ensure a certain margin between the point of operation of an aircraft engine and the surge zone. First of all, the idling of the engine may be increased, which increases the distance between the point of operation when idling and the surge zone, as explained previously. In addition, the engines may have relief valves, situated after the low-pressure compressor or the high-pressure compressor. The opening of these valves increases the flow of air in the compressor. Nevertheless, although these valves permit the thrust of the turbine engine to be maintained at a desired level, their use will involve a significant increase in consumption in order to obtain this thrust, as well as an increased risk of failure.
Another familiar approach, making it possible not to impact too negatively on the margin with regard to the surge zone, comprises or consists of a turbofan engine including a low-pressure compressor, an intermediate compressor and a high-pressure compressor, for taking off the power, via a transmission gearbox, at the level of a shaft connected to the intermediate compressor in place of a shaft connected to the high-pressure compressor, as is generally the case. This is not always possible, however, especially since many turbine engines that are used for the propulsion of aircraft do not include an intermediate shaft.
Concerning the idling, the solution generally used to define its speed involves taking into consideration the worst-case scenario, that is to say by considering the maximum mechanical takeoffs (in order to generate mechanical, electrical or hydraulic power), and minimum takeoffs of air. The definition of the idling speed also takes into account the phase of the most rapid acceleration that could follow, and during which a rapid increase in the pressure ratio takes place, while the flow does not increase instantaneously.
Thus, according to a strategy for the control of the turbine engine generally used in an aircraft, the turbine engine is controlled so that the line of operation of its compressor (defined by the succession of its points of operation), in particular high-pressure, maintains a surge margin taking into account a possible sudden acceleration of the turbine engine, the maximum level of the requirement for mechanical takeoff (even if this means setting a threshold for these requirements), and not taking into account the takeoffs of air (based on the hypothesis that these may be stopped at any moment). This leads to the adoption of a potentially high speed of idling.
Document US2014/0297155 describes a method for the management of an aircraft engine, in which the level of the mechanical takeoffs is continuously assessed in order for it to be capable of being taken into account, and is corrected (by limitation of the authorization to take off the power), in order to satisfy oneself that the engine is able to produce the necessary thrust, or remains within an admissible range of temperature. This document proposes additionally the possibility of taking into account these takeoffs in order to actuate so-called “variable geometry” devices of the engine, for example valves for guiding the air at the inlet to the compressor, or relief valves. This controlling of the means of variable geometry may be undertaken in order to take into account the margin with regard to the surge zone.
Nevertheless, the management of turbine engines, in particular as regards the takeoffs taken in the course of the operation of a turbofan engine or turboprop engine of an aircraft, may be further optimized.